Part 48

By M. R. TRUMBOWER, V. S.

ANATOMY AND PHYSIOLOGY OF THE HEART AND BLOOD VESSELS.

(Pls. XX and XXI.)

The heart is a hollow, muscular organ, situated a little to the left of the center of the chest. Its impulse is felt on the left side on account of its location and from the rotary movement of the organ in action. It is cone-shaped, with the base upward; the apex points downward, backward, and to the left side. It extends from about the third to the sixth ribs, inclusive. The average weight is about 7 to 8 pounds. In horses used for speed the heart is relatively larger, according to the weight of the animal, than in horses used for slow work. It is suspended from the spine by the large blood vessels and held in position below by the attachment of the pericardium to the sternum. It is inclosed in a sac, the pericardium, which is composed of a dense fibrous membrane lined by a delicate serous membrane, which is reflected over the heart; the inner layer is firmly adherent to the heart, the outer to the fibrous sac, and there is an intervening space, known as the pericardial space, in which a small amount of serum--a thin translucent liquid--is present constantly.

The heart is divided by a shallow fissure into a right and left side; each of these is again subdivided by a transverse partition into two compartments which communicate. Thus there are four cardiac cavities--the superior, or upper, ones called the auricles; the inferior, or lower, ones the ventricles. These divisions are marked on the outside by grooves, which contain the cardiac blood vessels, and are generally filled with fat.

The right side of the heart may be called the venous side, the left the arterial side, named from the kind of blood which passes through them. The auricles are thin-walled cavities placed at the base, and are connected with the great veins--the venae cavae and pulmonary veins--through which they receive blood from all parts of the body. The auricles communicate with the ventricles each by a large aperture, the auriculo-ventricular orifice, which is furnished with a remarkable mechanism of valves, allowing the transmission of blood from the auricles into the ventricles, but preventing a reverse course. The ventricles are thick-walled cavities, forming the more massive portion of the heart toward the apex. They are separated by a partition, and are connected with the great arteries--the pulmonary artery and the aorta--by which they send blood to all parts of the body. At the mouth of the aorta and at the mouth of the pulmonary artery is an arrangement of valves in each case which prevents the reflux of blood into the ventricles. The auriculo-ventricular valve in the left side is composed of two flaps, hence it is called the bicuspid valve; in the right side this valve has three flaps and is called the tricuspid valve. The flaps which form these valves are connected with a tendinous ring between the auricles and ventricles; and each flap of the auriculo-ventricular valves is supplied with tendinous cords, which are attached to the free margin and under-surface, so as to keep the valves tense when closed--a condition which is produced by the shortening of muscular pillars with which the cords are connected. The arterial openings, both on the right and on the left side, are provided with three-flapped semilunar-shaped valves, to prevent the regurgitation of blood when the ventricles contract. The veins emptying into the auricles are not capable of closure, but the posterior vena cava has an imperfect valve at its aperture.

The inner surface of the heart is lined by a serous membrane, the endocardium, which is smooth and firmly adherent to the muscular structure of the heart. This membrane is continuous with the lining membrane of the blood vessels, and it enters into the formation of the valves.

The circulation through the heart is as follows: The venous blood is carried into the right auricle by the anterior and posterior venae cavae. It then passes through the right auriculo-ventricular opening into the right ventricle, thence through the pulmonary artery to the lungs. It returns by the pulmonary veins to the left auricle, then is forced through the auriculo-ventricular opening into the left ventricle, which propels it through the aorta and its branches into the system, the veins returning it again to the heart. The circulation, therefore, is double, the pulmonary, or lesser, being performed by the right side, and the systemic, or greater, by the left side.

As the blood is forced through the heart by forcible contractions of its muscular walls, it has the action of a force pump, and gives the impulse at each beat, which we call the pulse--the dilatation of the arteries throughout the system. The contraction of the auricles is quickly followed by that of the ventricles, and then a slight pause occurs; this takes place in regular rhythmical order during health.

INTERIOR OF CHEST SHOWING POSITION OF HEART AND DIAPHRAGM.]

Heart.

Right half, red. Left half, blue. Auricles, at upper end. Ventricles, at lower end. Arteries, red. Veins, blue.

1. Left carotid artery.

2. Left jugular vein.

3. Portal system.

4. Vessels of the liver.

5. Arteries of the stomach. (Caeliac axis).

6. Vessels of the large intestine.

7. Vessels of the small intestine.

8. Artery of left kidney.

CIRCULATORY APPARATUS.]

The action of the heart is governed and maintained by the pneumogastric nerve (tenth pair of cranial nerves); it is the inhibitory nerve of the heart, and regulates, slows, and governs its action. When the nerve is cut, the heartbeats increase rapidly, and, in fact, the organ works without control. When the nerve is unduly irritated the holdback, or inhibitory force, is increased, and the heart slows up in the same measure. The left cavities of the heart, the pulmonary veins, and the aorta, or systemic artery, contain red or florid blood, fit to circulate through the body. The right cavities of the heart, with the venae cavae, or systemic veins, the pulmonary artery, contain dark blood, which must be transmitted through the lungs for renovation.

The arteries, commencing in two great trunks, the aorta and the pulmonary artery, undergo division, as in the branching of a tree. Their branches mostly come off at acute angles, and are commonly of uniform diameter in each case, but successively diminish after and in consequence of division, and in this manner gradually merge into the capillary system of blood vessels. As a general rule, the combined area of the branches is greater than that of the vessels from which they emanate, and hence the collective capacity of the arterial system is greatest at the capillary vessels. The same rule applies to the veins. The effect of the division of the arteries is to make the blood move more slowly along their branches to the capillary vessels, and the effect of the union of the branches of the veins is to accelerate the speed of the blood as it returns from the capillary vessels to the venous trunks.

In the smaller vessels a frequent running together, or anastomosis, occurs. This admits of a free communication between the currents of blood, and must tend to promote equability of distribution and of pressure, and to obviate the effects of local interruption. The arteries are highly elastic, being extensile and retractile both in length and breadth. During life they are also contractile, being provided with muscular tissue. When cut across they present, although empty, an open orifice; the veins, on the other hand, collapse.

In most parts of the body the arteries are inclosed in a sheath formed of connective tissue, but are connected so loosely that, when the vessel is cut across, its ends readily retract some distance within the sheath. Independently of this sheath, arteries are usually described as being formed of three coats, named, from the relative positions, external, middle, and internal. This applies to their structure so far as it is discernible by the naked eye. The internal, serous, or tunica intima, is the thinnest, and is continuous with the lining membrane of the heart. It is made up of two layers--an inner, consisting of a layer of epithelial scales, and an outer, transparent, whitish, highly elastic, and perforated. The middle coat, tunica media, is elastic, dense, and of a yellow color, consisting of nonstriated muscular and elastic fibers, thickest in the largest arteries and becoming thinner in the smaller. In the smallest vessels it is almost entirely muscular. The external coat, tunica adventitia, is composed mainly of fine and closely woven bundles of white connective tissue, which chiefly run diagonally or obliquely around the vessel. In this coat the nutrient vessels, the vasa vasorum, form a capillary network, from which a few penetrate as far as the muscular coat.

The veins differ from arteries in possessing thinner walls, less elastic and muscular tissue, and for the most part a stronger tunica adventitia. They collapse when cut across or when they are empty. The majority of veins are provided with valves; these are folds of the lining membrane, strengthened by fibrous tissue. They favor the course of the blood and prevent its reflux. The nerves which supply both the arteries and the veins come from the sympathetic system. The smaller arteries terminate in the system of minute vessels known as the capillaries, which are interposed between the termination of the arteries and the commencement of the veins. Their average diameter is about one three-thousandth of an inch.

DISEASES OF THE HEART AND BLOOD VESSELS.

In considering diseases of the heart we meet with many difficulties, depending much upon the position which this organ occupies in the animal. The shoulders cover so much of the anterior portion of the chest, and often in very heavy-muscled horses the chest walls are so thick that a satisfactory examination of the heart is attended with difficulty. Diseases of the heart are not uncommon among horses; the heart and its membranes are frequently involved in diseases of the respiratory organs, diseases of the kidneys, rheumatism, influenza, etc. Some of the diseases of this organ are never suspected by the ordinary observer during life, and are so difficult to diagnose with any degree of certainty that we will have to confine ourselves to a general outline, giving attention to such symptoms as may serve to lead to a knowledge of their existence, with directions for treatment, care, etc.